1. Field of the Invention
The present invention relates to a process for producing a urethane-modified polyisocyanurate foam. More particularly, it relates to a process for producing a urethane-modified polyisocyanurate foam, wherein a certain specific polyol is employed.
2. Description of the Prior Art
A polyisocyanurate foam is a hard foam having excellent heat resistance and flame resistance, which is obtainable by polymerizing a polyisocyanate compound in the presence of a foaming agent and an isocyanate trimerizing catalyst. However, such a foam is extremely brittle, and it is difficult to use it for a practical application. Therefore, it is common to use a small amount of a polyol together with the polyisocyanate compound to obtain a urethane-modified polyisocyanurate foam containing urethane bonds. The polyol is used usually in an amount of at least about 2 by the ratio in the number of isocyanate groups/hydroxyl groups. Usually, the smaller the amount of the polyol, the less the effectiveness for the improvement against brittleness. On the other hand, as the amount of the polyol increases, the heat resistance or the flame resistance deteriorates, and the flammability increases. The change of these physical properties varies also depending upon the type of the polyol. Accordingly, for the same amount, the polyol is desired to provide higher effectiveness for the improvement against brittleness, less deterioration of the heat resistance or flame resistance and higher fire retardancy. As known polyols, there may be mentioned polyhydric alcohols such as ethylene glycol, propylene glycol and trimethylol propane; polyalkylene glycols such as polyethylene glycol and polypropylene glycol; polyether polyols having relatively low molecular weights, which are obtainable by the addition of an alkylene oxide to an initiator such as a polyhydric alcohol, a polyhydric phenol or an amine; polyester polyols having relatively low molecular weights; and combinations of these low molecular weight polyols with polyether polyols having relatively high molecular weights. However, in many cases, such conventional polyols do not fully satisfy the required properties, and polyols having better functional properties are desired.
In many cases, polyols are required to provide not only the above-mentioned functional properties for the improvement of the physical properties of the foam, but also other functional properties. One of them is a question of the compatibility in the use of a halogenated hydrocarbon foaming agent. Polyols having relatively high hydrophilic properties, such as polyhydric alcohols, polyethylene glycols, polyether polyols having a high content of oxyethylene groups or polyester polyols, have relatively low compatibility with a foaming agent, and their mixtures with a foaming agent have a difficulty that they are likely to undergo phase separation. On the other hand, polyols having high hydrophobic properties such as polyether polyols having a high content of oxypropylene groups have good compatibility with foaming agents, but have a difficulty that the physical properties of the foam thereby obtained, such as the fire retardancy, are poor. As a means to solve these two problems, there has been proposed a method in which a polyester polyol and a polyether polyol obtainable by the addition of propylene oxide to a polyhydric alcohol are incorporated (Japanese Examined Patent Publication No. 22052/1982). However, in this method, the compatibility of the polyester polyol with the polyether polyol is low, and the foaming agent tends to be dissolved preferentially in the polyether polyol side. Accordingly, it can hardly be said that the above-mentioned problem of compatibility has been adequately solved.